As authorities scramble to establish what happened to Malaysia Airlines Flight 17 in Ukraine, U.S. officials on Friday pointed the finger toward pro-Russia rebels.

U.S. Ambassador to the United Nations Samantha Power told an emergency meeting of the U.N. Security Council that the plane was "likely downed by a surface to air missile ... operated from a separatist-held location in eastern Ukraine."

She said if pro-Russian separatists are responsible for shooting down the plane with a missile, it cannot be ruled out that Russia offered help to operate the system,

Separatist leaders also boasted on social media about shooting down the plane and later deleted those references, she said.

On board were 298 people, none of whom survived the crash, she said. Three were infants.

A preliminary classified U.S. intelligence analysis has concluded that the missile that hit Flight 17 most likely was fired by pro-Russian separatists inside eastern Ukraine, according to a U.S. defense official with direct access to the latest information.

The official declined to be identified because of the sensitivity of the information.

Power had tough words for Russia, saying it had not lived up to its commitments to ease tensions and halt the flow of weapons over the border to the rebels in Ukraine.

Ukrainian Prime Minister Arseniy Yatsenyuk earlier blasted the "terrorists" he blamed for shooting down Flight 17 over Ukraine a day earlier, with 298 people aboard.

He called on all governments to back the investigation and "to support the Ukrainian government to bring to justice all these bastards who committed this international crime."

Russia, Ukraine trade accusations

Since the Malaysia Airlines jet fell from the sky above eastern Ukraine on Thursday, Russia and Ukraine - which routinely uses the word "terrorists" to describe pro-Russian rebels - have traded blame and accusations.

"Terrorists have killed almost 300 persons with one shot," Ukrainian President Petro Poroshenko said Thursday. "Among them are women, children, citizens of different countries of the world."

Russian President Vladimir Putin pointed the finger back at Ukraine, blaming its recent tough military operations against separatists for the volatility in the region.

But Ukrainian Foreign Minister Pavlo Klimkin rejected that claim, telling CNN it was up to Russia to stop the flow of heavy weaponry across Ukraine's eastern border and push the separatists to embrace a cease-fire.

He also dismissed any suggestion that Ukrainian forces may have been involved in Thursday's tragedy.

"There was no way our forces could be engaged in any way in this incident," Klimkin said, adding that Ukraine did not have any military assets in the area that could have shot down MH17.

Klimkin says Ukraine intercepted telephone calls between "terrorists" at the time the plane was shot down.

Yatsenyuk called for a U.N. Security Council meeting to be held and for all nations to do everything they could to stop what he said was not now just a war in Ukraine or Europe, but a "war against the world."

Meanwhile, international inspectors headed to the crash site Friday tasked with finding the plane's flight data recorders, which may lie amid the human remains and debris strewn across fields near the town of Torez.

Ukrainian government officials said 181 bodies had been found.

The latest information from Malaysia Airlines indicates that the Netherlands has suffered the harshest blow, with at least 189 of its citizens among those killed.

Experts have voiced concern that the crash site has not been properly secured, making the recovery of bodies and collection of evidence difficult.

Former U.S. Deputy Defense Secretary Paul Wolfowitz, one of the original architects of the U.S. invasion of Iraq in 2003, told CNN's "New Day" that the United States "should have found a way to keep an American presence in Iraq."

American military forces ended their withdrawal from the country in December 2011. At the time of the U.S. drawdown, Iraq's leadership had agreed that a residual U.S. military presence was desirable, but talks broke down over the prickly issue of legal immunity for U.S. troops in Iraq.

Wolfowitz acknowledged that "a lot of mistakes" had been made in Iraq and that it was not "set up well for Obama when he came in," but he said Obama could have done more after taking power to renegotiate the terms of the U.S. withdrawal agreed on under former President George W. Bush.

"I think we could have kept a substantial, not a huge, American presence - not a combat presence, but the kind of support that would've kept Maliki better under control, that would've given the Iraqi army better ability to function," he said.

As the crisis in Ukraine shows no signs of easing, U.S. Vice President Joe Biden promised support for Ukraine and stressed that the United States won't recognize Russia's annexation of Crimea.

"Ukraine is and must remain one country," he said in Kiev on Tuesday at a news conference with Prime Minister Arseniy Yatsenyuk.

"No nation has the right to simply grab land from another nation," Biden said. "We will never recognize Russia's illegal occupation of Crimea."

Biden called on Russia to "stop supporting men hiding behind masks and unmarked uniforms sowing unrest in eastern Ukraine." He warned of additional sanctions if such "provocative behavior" does not end.

Ukrainian and U.S. officials say they think Russian special forces are in the region and are behind efforts to seize government buildings and generally promote unrest - a claim Moscow denies.

As well as voicing solidarity with Kiev, Biden promised financial support, assistance in reducing Ukraine's dependence on Russian energy sources and nonlethal aid for security forces.

"You will not walk this road alone. We will walk it with you," Biden told Yatsenyuk.

As he spoke, the White House announced a $50 million package of assistance to help Ukraine pursue political and economic reform and strengthen its partnership with the United States.

Biden said he also expects an International Monetary Fund package for Ukraine to be finalized imminently.

The backing is likely to sit well with Ukrainian leaders struggling to keep their country afloat amid dire financial problems, the ongoing showdown with Russia over its annexation of Crimea and alleged interference in Ukraine's pro-Russian east.

Tensions remain high as pro-Russian militants show no sign of leaving occupied government buildings in eastern Ukraine despite an international deal agreed to in Geneva, Switzerland, last week.

Ukraine's acting President Oleksandr Turchynov told the country's parliament Tuesday that an "anti-terrorist operation" is under way in part of Ukraine's eastern Donetsk region.

"It will be conducted step by step, responsibly, carefully," he said, according to the Ukrinform news agency.

The reported operation comes as EU defense ministers and NATO chief Anders Fogh Rasmussen meet in Luxembourg with the crisis in Ukraine high on the agenda.

Turchynov had given pro-Russian protesters in other eastern Ukrainian cities until 2 a.m. ET Monday to disarm or face a "full-scale anti-terrorist operation" by Ukraine's armed forces.

But the deadline passed with no sign that it was heeded, including in the eastern city of Donetsk, where protesters have held the regional government building for more than a week. Similar deadlines in the past have come and gone with no consequences.

A U.N. human rights report released Tuesday on the situation in Ukraine, including the Crimea region annexed by Russia last month, cited an urgent need to counter "misinformation, propaganda and incitement to hatred" in the country to avoid the further escalation of tension.

"Facts on the ground need to be established to help reduce the risk of radically different narratives being exploited for political ends," said U.N. High Commissioner for Human Rights Navi Pillay.

Based on information gathered by U.N. human rights monitors since March 15, the report draws concerning parallels between what happened in Crimea and events unfolding in eastern Ukraine now.